System and Method for Attitude Control of a Flight Vehicle using Pitch-Over Thrusters

ABSTRACT

A reliable and inexpensive attitude control system uses a plurality of pitch-over thrusters to perform rapid and precise attitude maneuvers for a flight vehicle. The pitch-over thrusters create rotational moments that directly pitch and yaw the flight vehicle. The use of very simple thrusters and control techniques provides for a reliable and cost effective solution. The ability to perform overlapping pitch and yaw maneuvers with single-shot fixed-impulse thrusters provides for high-speed maneuverability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of priority under 35 U.S.C. 120 as acontinuation application of co-pending U.S. patent application Ser. No.11/470,900 entitled “System and Method for Attitude Control of a FlightVehicle using Pitch-Over Thrusters” and filed Sep. 7, 2006, and under 35U.S.C. 119(e) to U.S. Provisional Application No. 60/780,895 entitled“All Aspect Pitchover Using Divert Thrusters” and filed on Mar. 7, 2006the entire contents of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system and method of attitude control forflight vehicles including missiles, kill vehicles and space craft.

2. Description of the Related Art

Attitude control systems are used to maneuver flight vehicles such as amissile, kill vehicle (KV) or space craft. One approach is to controlmoveable fins or other airfoils to produce the force vector required topitch and yaw the flight vehicle. This approach is relativelyinexpensive but is not capable of maneuvering the vehicle with bothspeed and precision, has moving parts that reduce reliability and willnot work in space. Another approach is to use active thrusters. Thisapproach is considerably more expensive and has its own reliabilityconcerns due to the complexity of the thrusters and closed-loop controlsystems. Active thrusters can produce larger force vectors to reduce theresponse time of any maneuver.

As shown in FIG. 1, a missile 10 is oriented with its body axis 12 alongthe x-axis. The missile's rocket motor 14 produces a thrust vector 16that ordinarily imparts a force on the missile along its body axis. Anarticulated nozzle 18 turns thrust vector 16 to impart a pitch (rotationabout the y-axis) or yaw (rotation about the z-ax) moment to themissile. An IMU 20 measures the attitude of the missile and providesactive feedback control to articulated nozzle 18. This is known as“Thrust Vector Control” and is typically used with tactical missilesduring flight to provide relatively gross and slow attitude control. Arelated approach is to fix nozzle 18 but place turning vanes inside thenozzle to turn the thrust vector. This is known as “Jet Vane Control”.Both techniques require moving parts, are relatively expensive solutionsand are generally incapable of executing tight turning radius maneuvers.

Another approach is to use a combination of roll-control thrusters andpitch-over thrusters as illustrated in FIGS. 2 a-2 c and 3 a-3 c. A pairof roll-control thrusters 30 and 32 are mounted on opposite sides of amissile 34 so that their thrust vectors produce a rotational moment that“rolls” the missile around its body axis in either direction. A pair ofpitch-over thrusters 36 and 38 are mounted on opposite sides of missile34 in the pitch plane and offset from the missile's center of gravity sothat their thrust vectors produce a rotational moment that “pitches” themissile. Because the missile's moment of inertia to pitch is muchgreater than its moment of inertia to roll, the pitch thrusters aregenerally capable of producing substantially more thrust than are theroll-control thrusters. Both types of thrusters are typically fixedamplitude and variable pulse-width to control the total applied force.Although the roll-control and pitch-over thrusters are shown on themissile orthogonal to each other, they can be aligned or have anydesired orientation with respect to each other because they actindependently.

To maneuver missile 34 from its current heading to an attitude 40, theroll-control thrusters 30 and 32 and than the pitch-over thrusters 36and 38 are fired in sequence. Because the “roll” and “pitch” maneuversare performed sequentially, any error in the “roll” maneuver will inducea large error in the “pitch”. Consequently, active closed-loop controlis used for both maneuvers. As shown in FIGS. 2 a and 3, roll-controlthruster 30 is fired producing thrust vector 42 that rolls the missilein a counter-clockwise direction. In an active closed-loop controlsystem, the IMU will constantly measure the roll angle and feed it backto the attitude controller, which in turn will either continue to firethruster 30 to continue rolling the missile or, if the missile hasrolled to far, fire thruster 32 to produce thrust vector 44 to roll itback. Once the roll maneuver has stabilized so that the plane ofpitch-over thrusters 36 and 38 is aligned with desired attitude 40,pitch-over thruster 36 is fired producing a thrust vector 46 thatrotates the missile around its center of gravity in the pitch plane. Theattitude controller actively controls pitch-over thrusters 36 and 38,which produce thrust vectors 46 and 48, to stabilize the orientation ofthe missile along attitude 40. Although this approach is widely used forattitude control in large, expensive missile systems, the attitudecontrol system is expensive and less reliable on account of thepulse-width modulated thrusters and active control system and relativelyslow on account of the sequential “roll” and “pitch” maneuvers.

In modern weapons systems and space craft demands are being placed onthe attitude control systems to be able to perform attitude maneuversvery quickly and precisely with high reliability and at low cost.Smaller and lower cost flight vehicles are being planned for deploymentin much higher volumes that are placing higher demands on performance atlower costs. The known approaches for attitude control cannot meet thecost-performance requirements of these systems.

SUMMARY OF THE INVENTION

The present invention provides a reliable and inexpensive attitudecontrol system that is capable of performing rapid and precise attitudemaneuvers for a flight vehicle.

The attitude control system includes at least four pitch-over thrusterson the flight vehicle and displaced from the vehicle center of gravityso that their thrust vectors produce rotational moments to directlypitch and yaw the flight vehicle. A fire controller is configured toissue start firing commands to a first set of the thrusters to slew theflight vehicle and to issue stop firing commands to a second set of thethrusters to stop the flight vehicle at a specified pitch and yaw tocomplete the attitude control maneuver.

In an exemplary embodiment, first and second pairs of pitch-overthrusters are placed on the vehicle in pitch and yaw planes,respectively. Each thruster suitably produces a single fixed-impulsethrust vector when fired. Multiple sets of pitch-over thrusters can beprovided if multiple attitude control maneuvers are required. Knowingthe inertial properties of the flight vehicle and the fixed-impulse ofthe thrust vectors, start and stop firing times can be calculated andstored in a look-up table for different attitude control maneuvers. Thefire controller looks up the start times for a particular maneuver andissues the start firing commands to one of the thrusters in each pair.If operating open-loop, the fire controller simply fires the otherthruster in each pair at the prescribed stop time. If closed-loop, thefire controller receives measurements of the pitch and yaw as thevehicle slews and makes adjustments to the prescribed stop times. Thepitch and yaw maneuvers can be performed sequentially or overlapped intime to reduce the response time of the attitude control maneuver.

Although this approach is applicable to provide attitude control for anyflight vehicle including all types of missiles, kill-vehicles and spacecraft, it is particularly applicable for smaller highly maneuverablecost-constrained missile systems. One such system is an “ActiveProtection System” or APS in which a missile system is integrated with avehicle such as a HumVee or armored personnel carrier to providedefensive counter-measures against possible attack. The missiles must beable to perform attitude control maneuvers very quickly and veryprecisely to overcome potential threats, must be highly reliable andinexpensive. A typical APS would include a threat detection system fordetecting and tracking a target and providing a launch command and pitchand yaw commands to the missile(s) for a desired attitude to the target.Each missile has a main thruster for providing thrust a long a body axisof the missile and at least four pitch-over thrusters displaced from themissile center of gravity whose thrust vectors produce rotationalmoments to directly pitch and yaw the missile. A launch thrusterlaunches the missile out of the vehicle upon receipt of the launchcommand. A fire controller receives the pitch and yaw commands andissues start firing commands to a first set of said thrusters to slewthe missile and to issue stop firing commands to a second set of saidthrusters to stop the missile at the specified pitch and yaw to achievethe desired attitude to the target. Once the missile is oriented at thetarget, the fire controller issues a command to ignite the main thrusterto fire the missile at the target. An implementation that usessingle-shot fixed-impulse thrusters and an open-loop fire controller ismuch less expensive and more reliable than current attitude controlsystems. The capability to actively pitch and yaw the missilesimultaneously allows the missile to complete the attitude controlmaneuver in no more than 200 ms and typically less than 100 ms.

These and other features and advantages of the invention will beapparent to those skilled in the art from the following detaileddescription of preferred embodiments, taken together with theaccompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, as described above, is a diagram illustrating Thrust VectorControl of a missile using an articulated nozzle;

FIGS. 2 a-2 c, as described above, are diagrams illustrating independentroll/pitch control of a missile;

FIG. 3 is a plot of a representative closed-loop firing sequence forindependent roll/pitch control shown in FIGS. 2 a-2 c;

FIGS. 4 a-4 b are diagrams illustrating attitude control usingpitch-over thrusters in accordance with the present invention;

FIGS. 5 a-5 b are diagrams illustrating the symmetry of the start andstop firing command times for two pair of fixed-impulse thrustersdisposed in pitch and yaw planes, respectively;

FIGS. 6 a-6 b are diagrams illustrating the construction of a thrusterfiring table for one region;

FIGS. 7 a-7 c are diagrams illustrating the start and stop firingcommands for fixed-impulse thrusters for a particular attitude controlmaneuver operating open-loop and closed-loop; and

FIGS. 8 a-8 c are diagrams of an active protection system (APS).

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a reliable and inexpensive attitudecontrol system that is capable of performing rapid and precise attitudemaneuvers for a flight vehicle. The system uses a plurality ofpitch-over thrusters to create rotational moments that directly pitchand yaw the flight vehicle. The use of very simple thrusters and controltechniques provides for a reliable and cost effective solution. Theability to perform overlapping pitch and yaw maneuvers with single-shotfixed-impulse thrusters provides for high-speed maneuverability.

In an exemplary embodiment, a first pair of pitch-over thrusters 102 and104 are mounted on a missile 106 in a pitch plane 108 and displaced fromthe missile's center of gravity 110. Similarly, a second pair ofpitch-over thrusters 112 and 114 are mounted on missile 106 in a yawplane 116 and displaced from the missile's center of gravity 110. Note,the pitch and yaw planes are defined with respect to the missile headingand roll angle and will change constantly. The terminology is used toconvey that the two pairs of thrusters are orthogonal and that at agiven roll angle and heading one pair will pitch the missile and theother pair will yaw the missile to perform the attitude controlmaneuver.

When fired, pitch-over thrusters 102 and 104 and 112 and 114 generatethrust vectors that produce rotational moments to directly pitch and yawthe missile, respectively. The two pair of orthogonally orientedpitch-over thrusters is the minimal and simplest configuration that candirectly pitch and yaw the missile. The thrust vectors preferablyintercept the missile body axis 118 so as not to induce roll. However,more complicated configurations with additional thrusters could beimplemented to perform the same function. To improve reliability andkeep costs low, each thruster is preferably very simple. For example, aconfiguration of fixed-impulse thrusters is highly effective andsimplifies attitude control. Because the moments of inertia in the pitchand yaw planes of a typical missile are similar, the pitch and yawthrusters may have the same fixed-impulse response. In manyapplications, single-shot fixed-impulse thrusters provide a highlyreliable and inexpensive solution to perform a single attitude controlmaneuver. To accommodate multiple control maneuvers, an additional setof pitch-over thrusters 120 can be mounted on the missile.

A fire controller 122 issues start firing commands to a first set ofsaid thrusters (e.g. thrusters 104 and 112) to slew the flight vehicleand to issue stop firing commands to a second set of said thrusters(e.g. thrusters 102 and 114) to stop the flight vehicle at the specifiedpitch and yaw to perform the attitude control maneuver. To stop theflight vehicle, the sum of the rotational moments in each of the pitchand yaw planes must be zero. Assuming the distance from the thruster tothe center of gravity is the same, the total impulse of the start andstop thrusters must be equal. To stop the flight vehicle at thespecified attitude (pitch, yaw), the timing of the firing sequence mustbe well controlled.

If the actual delays between start and stop thrusters are longer thanthe specified delays, the missile will over-rotate. Conversely, if theactual delays are shorter than the specified delays the missile willunder-rotate. The greater the duration between firing the start and stopthrusters the greater the missile will pitch or yaw.

Knowing the inertial properties of the missile and the fixed-impulse ofthe thrust vectors, start and stop firing times can be calculated andstored in a look-up table (LUT) 124 for different attitude controlmaneuvers. The fire controller looks up the start times for a particularmaneuver and issues the start firing commands to one of the thrusters ineach pair. If open-loop, the fire controller simply fires the otherthruster in each pair at the prescribed stop time. If closed-loop, thefire controller receives measurements of the pitch and yaw as thevehicle slews and makes an adjustment to the prescribed stop time. Thepitch and yaw maneuvers can be performed sequentially or simultaneouslyto reduce the response time of the attitude control maneuver. In analternate but equivalent embodiment, the LUT may store only a deltabetween the start and stop firing times. In this case, when the firecontroller receives a control command it simply fires all of the startthrusters and than at the respective deltas, fires the stop thrusters.An exemplary embodiment for constructing LUT 124 is illustrated in FIGS.5 a-5 b and 6 a-6 b. In this embodiment, it is assumed that each of thepitch-over thrusters produce the same fixed-impulse and rotationalmoments. These symmetry properties greatly streamline the calculation ofthe LUT and the amount of memory required to store the LUT. The missile106 and pitch-over thrusters T1 102, T2 114, T3 104 and T4 112 are shownin a coordinate frame 130, which is divided into regions 1, 2, 3 . . . 8covering 360° of yaw. Each region covers 45° of yaw and 180° of pitch inthis representation. A table 132 maps the thrusters that are used tostart and stop the pitch and yaw for each region. Region 1 covers a yawbetween 0 and 45° . In this region, thrusters T2 and T4 are fired to yawthe missile to the desired angle and thrusters T3 and T1 are fired topitch the missile to the desired angle. Because of the symmetryproperties, only the thruster firing sequences for missile attitudes inRegion 1 need be calculated. All other firing sequences are permutationsof the firing sequence used for Region 1. In cases where the missile isasymmetric it may be necessary to calculate thruster firing sequencesfor the full 360° of yaw.

The firing sequence for Region 1 is constructed by firing thrusters T1,T2, T3 and T4 with different (Δt1, Δt2) where Δt1=t3−t1 and Δt2=t2−t4which span Region 1 as shown in FIG. 6 a. Each data point (Δt1, Δt2) 134has an associated (Pitch, Yaw) that is stored but not shown. To beuseful the LUT should provide the firing sequence (Δt1, Δt2) for aspecified (Pitch, Yaw). This is accomplished by mapping the data points134 from Δt1vs. Δt2to pitch vs. yaw data points 136 as shown in FIG. 6b. To form the final LUT, these data points are interpolated to providea uniform grid at a desired resolution.

As mentioned above, the fire controller can operate either “open” or“closed” loop, which trade off simplicity and cost versus precision. Toexecute an attitude control maneuver 140 as shown in FIG. 7 a, thrustersT1 and T4 will be fired to pitch and yaw the missile, respectively.Thrusters T3 and T2 will be fired to stop the respective pitch and yawmaneuvers. As shown in FIG. 7 b, for open-loop control the firecontroller simply looks up the four start and stop times for T1, T2, T3and T4 and issues the firing commands accordingly. As shown each of thethrusters produces the same fixed-impulse thruster vector. In thisexample, the start time for T2 is offset from T1 but they could be firedsimultaneously. Because the attitude control maneuver 140 requires alarger pitch maneuver Δt1 is considerably longer than Δt2. In manysystems, the pulse width of the thrust vector is less than 50 ms andnominally 1-10 ms. The complete firing sequence typically takes lessthan 200 ms and nominally 100 ms. As shown in FIG. 7 c, for closed-loopcontrol 142, the guidance system or possibly the fire controllerreceives measurements of the pitch and yaw rates caused by firingthrusters T3 and T2 and revises the fire-time solution by adjusting thestop times for T1 and T4, respectively. These adjustments may be causedby non-uniformities in the thrusters or variations in the missile'smoments of inertia. As opposed to conventional closed-loop solutionsthat control the pulse-width of the thrusters and iteratively fireopposing thrusters, the proposed closed-loop solution makes a singleadjustment to the thruster stop time. This simplified closed-loopapproach can be used with the single-shot fixed-impulse thrusters.

Although this approach is applicable to provide attitude control for anyflight vehicle including all types of missiles, kill-vehicles and spacecraft, it is particularly applicable for smaller highly maneuverablecost-constrained missile systems. One such system is an “ActiveProtection System” or APS 150 shown in FIG. 8 a in which a missilesystem is integrated with a vehicle 154 such as a HumVee or armoredpersonnel carrier to provide defensive counter-measures against possibleattack. The missiles 152 must be able to perform attitude controlmaneuvers very quickly and very precisely to overcome potential threats,must be highly reliable and inexpensive. A typical APS would include themissile 152, a threat detection system (TDS) 156 for detecting andtracking a target and providing a launch command and pitch and yawcommands to the missile(s) for a desired attitude to the target, and alaunch thruster 158 to launch the missile out of the vehicle uponreceipt of the launch command Each missile 152 has a main thruster 160for providing thrust a long a body axis of the missile, at least fourpitch-over thrusters 162 displaced from the missile center of gravitywhose thrust vectors produce rotational moments to directly pitch andyaw the missile, and a fire controller to issue the firing commands

As shown in FIG. 8 b, in an exemplary embodiment threat detection system156 includes a threat detection sensor 166, a threat tracking system 168and fire-time solution system 170. Threat detection sensor 166 sends a“begin track command” and its sensed data to threat tracking system 168,which analyzes the data and outputs a “threat state”. This processcontinues until a launch decision routine 172 decides that a real threatexists and a missile 152 should be launched to counter the threat. The“threat state” is passed to the fire-time solution system 170, whichdetermines which available missile to launch, when to launch, when toinitiate the pitch-over maneuver and the specific start and stopcommands for the firing sequence. The fire-times to initiate pitchoverand the specific start and stop commands are loaded into the missile'sfire control system.

As shown in FIG. 8 c, at T=0 the Fire-Time Solution System 170 issuesthe launch command causing launch thruster 158 to ‘pop’ missile 152 upabove the vehicle. Once the pitch-over height is reached (T=0.02 s), afire controller 164 issues the start firing commands to a first set ofsaid thrusters to pitch and yaw the missile (T=0.25 s) and issues thestop firing commands to a second set of said thrusters to stop themissile at the specified pitch and yaw to achieve the desired attitudeto the target (T=0.035 s). If closed-loop control is employed, theadjustment to the stop times must be made within the window of about 100ms. Once the missile is oriented at the target, the fire controllerissues a command to ignite the main thruster to initiate boost to firethe missile at the target (T=0.4). Boost is completed at T=0.6 secondswith intercept of target 174 occurring at a range of approximately 100feet in about 0.16 seconds from launch to intercept. An implementationthat uses single-shot fixed-impulse thrusters and an open-loop firecontroller is much less expensive and more reliable than currentattitude control systems. The capability to actively pitch and yaw themissile simultaneously allows the missile to complete the attitudecontrol maneuver in no more than 200 ms and typically less than 100 ms.

While several illustrative embodiments of the invention have been shownand described, numerous variations and alternate embodiments will occurto those skilled in the art. Such variations and alternate embodimentsare contemplated, and can be made without departing from the spirit andscope of the invention as defined in the appended claims.

1. An attitude control system for a flight vehicle, comprising: at leastfour thrusters on the flight vehicle and displaced from the vehiclecenter of gravity so that their thrust vectors produce rotationalmoments to directly pitch and yaw the flight vehicle; and a firecontroller configured to operate open-loop to issue start firingcommands to a first set of said thrusters to slew the flight vehicle andwithout measurement of the pitch and yaw as the vehicle slews to issuestop firing commands to a second set of said thrusters to stop theflight vehicle at a specified pitch and yaw.
 2. The attitude controlsystem of claim 1, wherein the flight vehicle is selected from amissile, kill vehicle (KV) or space craft.
 3. The attitude controlsystem of claim 1, wherein each said thruster generates a thrust vectorwith a fixed impulse.
 4. The attitude control system of claim 1, whereinsaid at least four thrusters includes a first pair of thrusters whosethrust vectors are collinear in a pitch plane and a second pair ofthrusters whose thrust vectors are collinear in a yaw plane.
 5. Theattitude control system of claim 1, wherein the fire controller stores atable of start and stop times for a plurality of yaw and pitch maneuversfor issuing the start and stop firing commands.
 6. The attitude controlsystem of claim 1, wherein fire controller issues the start firingcommands so that the pitch and yaw slewing overlap at least partially intime.
 7. The attitude control system of claim 1, wherein the thrustvectors produce rotational moments to directly pitch and yaw the flightvehicle in pitch and yaw planes, respectively, said fire controllerconfigured to issue the start firing commands to the first set of saidthrusters to slew the flight vehicle and to issue the stop firingcommands to the second set of said thrusters so that the sum of therotational moments in each of the pitch and yaw planes is zero to stopthe flight vehicle at a specified pitch and yaw.
 8. The attitude controlsystem of claim 7, wherein all of the thrusters are displaced the samedistance from the center of gravity, the total impulse of the first setof thrusters is equal to the total impulse of the second set ofthrusters.
 9. An active protection system, comprising: a threatdetection system for detecting and tracking a target and providing alaunch command and pitch and yaw commands for a desired attitude to thetarget; a missile having a main thruster for providing thrust a long abody axis of the missile and at least four thrusters displaced from themissile center of gravity whose thrust vectors produce rotationalmoments to directly pitch and yaw the missile; a launch thruster tolaunch the missile upon receipt of the launch command; and a firecontroller on the missile configured to operate open-loop to issue startfiring commands to a first set of said thrusters to slew the missile andwithout measurement of the pitch and yaw as the vehicle slews to issuestop firing commands to a second set of said thrusters to stop themissile at a specified pitch and yaw to achieve the desired attitude tothe target, said fire controller than issuing a command to ignite themain thruster to fire the missile along the desired attitude to thetarget.
 10. The active protection system of claim 9, wherein thethrusters generate thrust vectors with a fixed-impulse.
 11. The activeprotection system of claim 9, wherein said at least four thrustersincludes a first pair of thrusters whose thrust vectors are collinear ina pitch plane and a second pair of thrusters whose thrust vectors arecollinear in a yaw plane.
 12. The active protection system of claim 9,wherein the fire controller stores a table of start and stop times for aplurality of yaw and pitch maneuvers for issuing the start and stopfiring commands.
 13. The active protection system of claim 9, whereinsaid at least four thrusters includes a first and second pairs ofthrusters that slew the missile in pitch and yaw planes, respectively,said look-up table storing one start and one stop time for each pair.14. The active protection system of claim 9, wherein fire controllerissues the start firing commands so that the pitch and yaw slewingoverlap at least partially in time.
 15. The active protection system ofclaim 9, wherein the thrust vectors produce rotational moments todirectly pitch and yaw the missile in pitch and yaw planes,respectively, said fire controller configured to issue the start firingcommands to the first set of said thrusters to slew the missile and toissue the stop firing commands to the second set of said thrusters sothat the sum of the rotational moments in each of the pitch and yawplanes is zero to stop the missile at a specified pitch and yaw.
 16. Theactive protection system of claim 9, wherein all of the thrusters aredisplaced the same distance from the center of gravity, the totalimpulse of the first set of thrusters is equal to the total impulse ofthe second set of thrusters.